Cellular ceramic articles with coated channels and methods for making the same

ABSTRACT

Cellular ceramic articles are manufactured from a green cellular ceramic body that includes a binder material and a plurality of channels. At least one of the channels is coated with a slurry that includes a green coating composition and a solvent to form a coating layer. The binder material is insoluble in the solvent.

CROSS-REFERENCE To RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/219,016 filed on Aug. 26, 2011 which claims priority to ProvisionalApplication Ser. No. 61/378,486 filed on Aug. 31, 2010 the content ofwhich is relied upon and incorporated herein by reference in itsentirety.

BACKGROUND

The disclosure relates generally to cellular ceramic articles and moreparticularly to cellular ceramic articles having channels that arecoated with a slurry material while in a green state.

Diesel particulate filters and catalyst substrates made of extrudedhoneycomb ceramics are key components in modern engine after treatmentsystems designed to meet current and future emission legislation.Cordierite is currently the dominant material of choice for substratesand is used for diesel particulate filters as well, especially forheavy-duty applications. Cordierite-based filters are also consideredfor gasoline particulate filters, should future emission standardsrequire the need for such. Other material choices include stabilizedaluminum titanate (AT) and silicon carbide (SiC), either re-crystallizedor Si-bonded.

Such products are typically manufactured by an extrusion processfollowed by drying and high temperature thermal treatment processes(firing). For filters, an additional step is required to plug honeycombchannels. Cordierite and aluminum titanate honeycombs are syntheticceramics for which the extrusion batch primarily comprises precursormaterials such as alumina, silica, titania, etc. that react during thefiring step to form the finished ceramic. Additional components areadded to adjust rheological properties and to aid formation of poreswith the desired structure. As the final material is obtained only afterchemical reaction of the raw materials during the high temperaturethermal treatment, prior to that treatment, the honeycomb structures andbatch material are usually referred to as being in the “green” state.

In addition to possible material differences, the honeycomb structuresused today for filters and substrates also differ in the number of cellsper unit area, typically expressed as cells per square inch (cpsi), theweb thickness, and the porosity characteristics of the wall material,namely porosity and pore size distribution. All of the extruded productscommercially manufactured today have essentially uniform porosity andpore size distribution along the wall from inlet to outlet face andacross the webs from one channel to the adjacent channel, with theporous characteristics determined primarily by the composition of thegreen batch and the subsequent thermal treatment steps. Furthermore,such products have essentially constant web thickness from inlet tooutlet, as determined by the dimensions of the extrusion die. Incontrast, products with varying web thickness in the radial direction toincrease mechanical strength, i.e. the web thickness increases from thecenter to the skin, are commercially available. This variability istypically designed through different slot sizes of the extrusion die andagain does not change along the main axis of the part.

For applications requiring only a substrate (where no channels areplugged as they are in diesel particulate filters), a catalyticallyactive material is disposed on the substrate, typically via awashcoating process. In this process, the catalytically active materialis applied in the form of a slurry, with the catalyst materials beingdispersed and dissolved therein. Driven by a slip casting effect, thecatalyst particles are deposited primarily onto the geometric surface ofthe substrate with some portion actually penetrating into the substratepore structure and acting as anchors to provide good adhesion betweenthe coating and substrate walls. To increase the degree of adhesion, aweb surface with high porosity and a tailored pore size is desirable.However, to prevent excessive penetration of the coating into the wall,where the catalyst utilization would be lower due to diffusionlimitations, a web surface with lower porosity and finer pores isdesirable. In addition, a very low porosity substrate has an advantagein mechanical strength.

In the case of soot filtration inside a so-called wall flow filter, asused on diesel engines today, the pressure drop increases as sootbecomes trapped in the filter walls. This is undesirable from bothengine operation and fuel economy perspectives. To manage the overallpressure drop of the system the filter is frequently exposed(regenerated) to conditions during which the accumulated carbon-basedmatter is oxidized. In general, pressure drop is determined by thegeometry of the honeycomb in terms of hydraulic diameter of thechannels, open area for flow and web thickness and geometric orfiltration area. In addition, in the presence of soot the pressure dropincreases due to the amount of soot that penetrates into themicrostructure (deep bed filtration) as well as the amount of soot thataccumulates on the filter wall surface (cake filtration). Due to flowrestrictions in the porous wall and higher specific velocities, theimpact of deep bed soot (deposited inside the porous wall) on pressuredrop is significantly more pronounced compared to soot deposited ascake. It has been observed that this effect is reduced when the poresize is reduced, typically below a mean pore size of ˜10 μm. A drawbackto decreasing the pore size is that the wall permeability, even withoutsoot, decreases proportionally to the square of the pore size andlinearly with wall thickness. Accordingly, a thin surface layer withboth small pore size and high porosity supported by a substrate withlarge pore size and high porosity would serve to address at least someof these concerns.

As described above, the increase in pressure drop with accumulation ofsoot requires frequent regeneration of the filter and removal byoxidation of the accumulated soot. Under certain conditions, referred toas uncontrolled regeneration, the heat release during this oxidationstep can be significant, resulting in an increase in the temperatureinside the filter. In extreme cases, this can lead to filter damage dueto thermal stresses or even melting. For filter materials, a strongcorrelation between the volumetric heat capacity (bulk density xspecific heat capacity) and the peak temperature observed during extremesoot regeneration events has been found. For high values of thevolumetric heat capacity, lower temperatures are observed. As a result,for a given material with a given specific heat capacity (J/kgK) and agiven maximum temperature, a higher bulk density is required forincreased soot mass limit. The latter can be achieved by either using alower porosity material or designing a filter with lower open channelvolume, i.e. thicker webs. In filter applications, the highesttemperatures are usually observed at the filter exit, so having a higherdensity at the exit would mitigate the increases in temperature. Withrespect to pressure drop, however, filters with higher porosity andthinner walls are desirable. Analogous to the trade off described abovefor substrates, the filter designs must be optimized to balance theseopposing characteristics, however such designs have not been shown to beeconomically obtained via a continuous extrusion process.

Catalytically active materials are now being coated not only onsubstrates but on some filters as well. The catalytic coating of pluggedparticulate filters typically is found inside the porous wall structure.This is in many cases desirable from a permeability perspective andoften driven by the coating process in which the slurry is forced toflow through the walls due to the alternate plugging pattern of thefilter channels. A common limitation is that any separation of catalystfunctionality, i.e. due to the presence of more than one type ofcatalytic active material, across the web or wall is technicallydifficult to achieve. Having an asymmetric pore structure with smallpores on one side of the wall would help to sieve/slip-cast the catalystparticles of a slurry applied from this side of the wall, preventingsubstantial penetration into the pore structure. An additional catalystmaterial could be applied from the other side of the web with resultingdeposition for example into the porous wall structure. With currentfilter products of homogenous pore size and pore structure across theweb, this is challenging at best, if not impossible.

The above application examples, although not exhaustive, demonstrate theneed for substrate and filter substrate bodies with webs that havedifferent properties either along the web from inlet to outlet face oracross the web from one channel to the adjacent channels. However, suchdesigns cannot be obtained in an economically viable manner via thecontinuous extrusion process. Existing methods to generate structureswith such variability on a web scale are based on applying a slurry,analogous to the catalyst coating process described above, to the firedsubstrate body. These methods, however, require additional thermaltreatment steps, generally create an interface with differentthermo-mechanical properties that will result in thermal stresses, andhave lower permeability as the pore structures are not continuous butrather in separate layers. The latter can be addressed to some extent byusing a multitude of layers with a gradient in properties but this comesat a high manufacturing cost.

SUMMARY

One embodiment of the disclosure relates to a method of manufacturing acellular ceramic article. The method includes providing a green cellularceramic body that includes a binder material and a plurality ofchannels. In addition, the method includes coating at least one of theplurality of channels with a slurry comprising a green coatingcomposition and a solvent to form a coating layer on at least one of theplurality of channels. Preferably, the binder material is insoluble inthe solvent and the at least one of the plurality of channels isunplugged when coated with the slurry.

Another embodiment of the disclosure relates to a green cellular ceramicbody. The green cellular ceramic body includes a binder material and aplurality of channels. A coating layer is on at least one of theplurality of channels. The coating layer is formed from a slurry thatincludes a green coating composition and a solvent. Preferably, thebinder material is insoluble in the solvent and the at least one of theplurality of channels coated with the coating layer is unplugged.

Yet another embodiment of the disclosure relates to a cellular ceramicarticle fired from a green cellular ceramic body. The green cellularceramic body includes a binder material and a plurality of channels. Acoating layer is on at least one of the plurality of channels. Thecoating layer is formed from a slurry that includes a green coatingcomposition and a solvent. Preferably, the binder material is insolublein the solvent and the at least one of the plurality of channels coatedwith the coating layer is unplugged.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C schematically illustrate coating a coating composition ontothe walls of fired ware as opposed to the walls of green ware;

FIG. 2 plots rate of weight loss per hour for green ware soaked insolvent compositions having differing amounts of water content;

FIG. 3 schematically illustrates a flow diagram of a manufacturingprocess integrating coating processes described herein;

FIGS. 4A-4B show two optical images illustrating aspects of the coatingof channels of a green cellular ceramic article according to processesdisclosed herein;

FIGS. 5A-5F show SEM images of cellular ceramic articles after firing,wherein a surface region on the coating layer has a porosity distinctfrom the that of bulk ceramic;

FIGS. 6A-6B show SEM images of two samples after firing, wherein asubstrate was and was not coated with a coating layer;

FIGS. 7A-7C show SEM surface images of fired ceramics for a baresubstrate, a coated substrate with no added pore former, and a coatedsubstrate with pore former;

FIG. 8 illustrates noise to signal ratio (N/S) upon soot loading;

FIGS. 9A-9B plot pressure drop and N/S data for different pluggedDPF-style filters;

FIGS. 10A-10B plot pressure drop and N/S data for plugged DPF-stylefilters having coated and uncoated channels;

FIG. 11 shows filtration efficiency as a function of soot-loading forcoated and uncoated DPF filters; and

FIG. 12 plots survivability test results for different plugged DPF-stylefilters having channels that have been coated with varying amounts ofcoating.

DETAILED DESCRIPTION

Disclosed herein is a novel process for tailoring the properties ofporous cellular ceramic articles. The process includes applying a thingreen coating to the walls of a green cellular ceramic and subsequentlyfiring the coated ware to convert both the coating and the wall to aporous ceramic filter. The green cellular body is preferably an extrudedhoneycomb comprising inorganic precursors, organic and inorganicbinders, pore formers, oil, and water. The green coating is preferably amixture of a suitable liquid vehicle, selected from among liquids thatdo not compromise the green substrate body, inorganic precursors andoptional pore formers. Examples of suitable liquid vehicles includealcohols having an acceptable properties, such as an acceptable degreeof hydrophobicity and volatility, although other suitable liquids canalso be used. The other inorganic and organic raw materials of the greencoating can be similar in nature or identical to those used in themaking of the green substrate body. In addition, similar technicalapproaches can be applied to tailor the properties of the coating, suchas for example its pore size and porosity. When the green coating andsubstrate are fired, the precursors and pore formers present in bothwill react and/or burn off, leading to a porous ceramic body with awell-connected solid phase.

As the coating is applied in the green state, while the walls of thegreen ware have low porosity with small diameter compared to a firedbody, formation of at least two relatively discrete regions across thewall can be formed, one determined by the green composition of thesubstrate material and the other determined by the green composition ofthe coating. A higher number of regions can be observed if, for example,both sides of the channel wall are coated or if multiple coatings ofdifferent composition are applied. Because the reaction preferablyoccurs simultaneously with sintering, all regions are enabled to bewell-connected and continuous in terms of the solid phase. The porespace in both regions can also be very well connected, as the gaseousproducts from the pore forming additives have to escape through thecoating layer.

Spatial properties of the resulting fired coated ceramic can besignificantly altered by the selection of the composition of the greencoating. For example, through suitable selection of particle size andbatch materials, coatings with higher or lower pore size and/or porosityor even different chemical composition compared to the base substratematerial region can be formed. Adding the coating in the green state cannot only improve the properties and uniformity of this coating, but canalso significantly decrease the cost and complexity in manufacturinginorganic membranes for at least the reason that only a single firingstep is required. In comparison, conventional methods for manufacturinginorganic membranes typically consist of multiple coating, drying andfiring steps.

Providing monolithic substrate materials with properties that varyeither across the wall or along the wall can address one or moreproblems found today in many applications. For example, the formation ofa thin surface layer with smaller pore size and equal or higher porositythan the bulk wall may result in a beneficial decrease in deep bedfiltration and a reduction in soot loaded pressure drop of a dieselparticulate filter (DPF). In addition, such a layer may increase thefiltration efficiency with minimal effect on backpressure. This thinsurface layer can also facilitate the deposition of an on-wall catalyticcoating for a DPF. The formation of a surface layer whose properties(porosity, pore size, and/or thickness) vary along the filter lengthcould also be provided. For example, a filter whose wall thicknessesincreased along the length could have reduced thermal axial gradients.Other examples for substrates include cases where a low porosity, highstrength substrate is coated with a thin layer of higher porosity, poresize and surface roughness to increase adhesion of a catalyst coatingapplied to it.

When a percentage increase or decrease is disclosed herein, thepercentage increase or decrease is to be understood as being relative tothe size of the un-increased or decreased parameter being referenced.For example, if a wall or channel being coated by a coating layer has asurface porosity of 30%, the statement “the surface porosity of thecoating layer is at least 5% greater than the surface porosity of a wallon the channel on which it is coated” is to be understood as describinga coating layer having a surface porosity of at least 1.05×30% and notat least 35%.

Embodiments disclosed herein provide a process applied to green cellularceramics that can modify particular properties in the resulting firedware. Articles made by this process can also be provided. Coating greenware can have one or more advantages when compared to coating firedware, particularly when the fired ware is substantially porous. A commonproblem with coating fired ware is that the particles that make up thecoating can penetrate into the substrate if the pores are large enough.The challenge with this approach is illustrated schematically in FIG.1A, where the particles 12 from the coating penetrate into the substratepore structure 10, significantly decreasing the overall wallpermeability.

The conventional solution to this problem is to decrease the particlesizes stepwise in layers wherein smaller particles 12 are layered overlarger particles 14, as shown in FIG. 1B, such that limited penetrationoccurs during any coating step. The material stack culminates in a toplayer where the particle size and sintering conditions are chosen toachieve the final pore size and porosity. Each of the coating steps aretypically performed at least twice to ensure that a continuous layerwith the smaller particles is formed before the next smallest sizeparticles are deposited. However, in the case of ceramic materials, thisis an extremely costly process approach as multiple coating, drying, and(high temperature) firing steps are required.

By coating relatively non-porous green ware 16, as shown in FIG. 1C, theparticles 12 making up the slurry can be as small as desired since thereis little-to-no porosity for the slurry to penetrate, resulting in asubstantial decrease in cost and complexity of manufacturing. Anotheradvantage is improved adhesion between the coating and the substratebecause the ware and coating are both green, during firing theprecursors react both within the coating and wall layers as well asacross the interface to form the final product. Yet another advantage isthat the coating that is formed is an on-wall coating, as the green waretypically has low porosity and very small pore diameter. This enablesits use as a barrier layer, which may be very useful for forming on-wallcatalyst coatings among other applications.

The solvent vehicle for the inorganic and pore former phases should bechosen such that the binder material in the green ware walls, forexample, methylcellulose, is insoluble in the solvent. For example, ifan aqueous-based slurry is used to apply a coating, the water in theslurry will dissolve some of the methylcellulose binder from the wall.To illustrate this point, slurries containing five different solventswere prepared and applied to cellular green ware. The first two solventswere alcohols, specifically butanol and isopropyl alcohol (IPA). Thenext three solvents were alcohol-water mixtures, specifically, isopropylalcohol and 10% water, isopropyl alcohol and 25% water, and isopropylalcohol and 50% water. As shown in FIG. 2, as the water content of thesolvent is increased, the weight loss of the green ware in wt % per houris also increased, which can lead to a significant weakening of theultimate product.

Accordingly, provided herein is a method of manufacturing a cellularceramic article that includes providing a green cellular ceramic body,the green cellular ceramic body comprising a binder material and aplurality of channels. The method also includes coating at least one ofthe plurality of channels with a slurry comprising a green coatingcomposition and a solvent to form a coating layer on at least one of theplurality of channels. The binder material should be insoluble in thesolvent. In addition, in preferred embodiments, the at least one of theplurality of channels is unplugged when coated with the slurry.

Suitable solvent components may include, for example, alcohols, esteralcohols, esters, hydrocarbons, aldehydes, ketones, and carboxylicacids. Preferably, the solvent used in the slurry comprises at least oneprimary, secondary or tertiary alcohol. Examples of alcohols that can beused as a solvent include methanol, ethanol, propanol, butanol,petnanol, and hexanol. An example of an ester alcohol that can be usedis Texanol (2,2,4-Trimethyl-1,3-pentanediol monoisobutyrate) and anexample of an ester that can be used is Optifilm Enhancer 300 (Propanoicacid,2-methyl-,1,1′-[2,2-dimethyl-1-(1-methylethyl)-1,3-propanediyl]ester), both soldcommercially by Eastman Chemical Company.

The green coating composition used in the slurry preferably includesinorganic precursors. The choice of inorganic precursors is dependentupon the desired composition. For example, the green coating compositionmay comprise materials such as alumina, titania, silica, strontiumcarbonate, calcium carbonate, and/or lanthanum oxide. The materialsselected for the green coating composition can be the same or differentfrom those chosen to form the ceramic composition in the bulk wall ofthe green cellular ceramic body, for example a green cellular ceramicbody in which aluminum titanate is the main phase and a feldspar is asecondary phase. Pore former may be optionally added to the greencoating composition to generate coating porosity. Although the amountand type of pore former can be varied depending on the desired porosity,preferred embodiments of pore formers include, for example, starchderived from potato, rice, and/or corn at about, for example, 1-50 wt %super-addition.

As discussed above, the green cellular ceramic body can, in one or moreembodiments, comprise materials that upon firing react to form aluminumtitanate (AT). However, the green cellular ceramic body is not limitedto materials that react to form AT and can comprise any materials ormixtures of materials that upon high temperature treatments react toform oxide or non-oxide ceramics, including metals, intermetallics,mullite, cordierite, alumina (Al₂O₃), zircon, alkali and alkaline-earthalumino-silicates, spinels, persovskites, zirconia, ceria, siliconcarbide (SiC), silicon nitride (Si₃N₄), silicon aluminum oxynitride(SiAlON), and zeolites.

A preferred binder material in the green cellular ceramic body is acellulose ether. Examples of preferred cellulose ethers includemethylcellulose and hydroxypropyl methylcellulose, including theMethocel family of products available from the Dow Chemical Company.Preferred binder materials may also include polyols, such aspolyvinylalcohol (PVA).

In one set of exemplary embodiments, the green cellular ceramic body andthe green coating composition can have the same or essentially the sameingredients. In another set of exemplary embodiments, the green cellularceramic body and the green coating composition can have at least somedifferent ingredients. In yet another set of exemplary embodiments, thegreen coating composition can comprise two or more different coatingcompositions. For example, the green coating composition may comprise afirst coating composition having the same or essentially the sameingredients as the green cellular ceramic body and a second coatingcomposition having at least some different ingredients than the greencellular ceramic body. Alternatively, the green coating composition maycomprise two or more coating compositions that each have at least somedifferent ingredients than the green cellular ceramic body. The two ormore coating compositions may be coated on different channels of theceramic, such as a first coating composition on inlet channels and asecond coating composition on outlet channels.

For example, the green coating composition may include at least onematerial that upon firing reacts to form at least one material selectedfrom the group consisting of aluminum titanate (AT), metals,intermetallics, mullite, cordierite, alumina (Al₂O₃), zircon, alkali andalkaline-earth alumino-silicates, spinels, persovskites, zirconia,ceria, silicon carbide (SiC), silicon nitride (Si₃N₄), silicon aluminumoxynitride (SiAlON), and zeolites, which can be the same or differentfrom the material of the green cellular ceramic body. It should be notedthat if the green coating composition is different from that of thegreen cellular ceramic body it should preferably be thermodynamicallystable with the green cellular ceramic body upon firing or at least bekinetically limited at the sintering temperature.

The green coating composition may also include a binder material. Thebinder material should preferably be selected to provide green strengthto the coating after drying and should preferably be dispersible in anyliquid vehicle used in the green coating composition. Preferredmaterials for the green coating composition binder material includecolloidal boehmite (AlOOH), colloidal silica, colloidal titania,tetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS),aluminum alkoxide, titanium alkoxide, and polyvinyl butyral, althoughany binder that is soluble or dispersable in the liquid vehicle may beused. Modification of the surface chemistry of colloidal binders such asboehmite or silica may be necessary to ensure good dispersion in thesolvent of choice. It is important to note that when using a bindermaterial that converts to inorganic material upon heating, the greenslip chemistry should preferably be adjusted to account for thisadditional inorganic to achieve the desired fired chemistry (e.g., AT,mullite, etc.).

Prior to coating channels of the green cellular ceramic body with theslurry, one or more of the plurality of channels may be masked such thatthe slurry is coated only on the channel or channels that are notmasked. For example, in one set of preferred embodiments, channelsintended to be plugged as outlet channels may be masked prior to coatingthe slurry such that slurry is coated only on channels intended to beplugged as inlet channels. Alternatively, channels intended to beplugged as inlet channels may be masked prior to coating the slurry suchthat slurry is coated only on channels intended to be plugged as outletchannels. In addition, channels may be masked such that different slurrycompositions are coated on different channels and/or different amountsof slurry compositions are coated on different channels.

The application of coating can be accomplished by pouring the slurryover the top face of the green cellular ceramic body and allowinggravity to pull the slurry down the channels (waterfall method). Theexcess slurry in the channels can be forced out the ends of the monolithby pressurized air to distribute the coating along the axial direction,and to remove any material that may lead to channel plugging.

The application of coating can also be accomplished by dipping the greencellular ceramic body in the slurry. For example, in embodiments wherethe thickness of the coating layer is intended to vary along the axiallength of at least some of the channels, the green cellular ceramic bodycan be dipped in the slurry for varying times and/or lengths along itsaxial length.

Additional methods of applying the coating can include using a vacuum topull the slurry on the channels, and pumping the slurry onto thechannels.

FIG. 3 schematically illustrates a process flow diagram showing anexample of how coating processes described herein (shown in shadedboxes) can be integrated into a broader manufacturing process. Once theslurry is prepared 110, for example, from raw materials similar to thosealready existing in the process (e.g., from an inorganic package 108,pore former package 106, and slurry liquid 104), the coating step can beinserted as a step between drying 100 and firing 116 steps. In theexemplary process shown in FIG. 3, green cellular ceramic articles arecoated with the slurry 112 and then dried in a dedicated drying step 114which optionally recovers the slurry liquid 104 by methods well known inthe art, such as condensation, adsorption, or absorption processes. Therecovered liquid may also be recycled back 102 to the slurry preparationstep. The coated and dried green substrate can then be fired in a firingprocess 116, according to methods known to those of ordinary skill inthe art. Following the firing process, the resulting cellular ceramicarticle may be plugged (in the case of filters) and finished 118according to methods known in the art.

FIGS. 4A and 4B show two optical images illustrating aspects of thecoating of channels of a green cellular ceramic article according toprocesses disclosed herein. FIG. 4A shows an end-on view of a greencellular ceramic sample which has been masked to block the outletchannels from coating (a continuous mask was applied to the endface andthen a laser was used to burn off the masking material covering theinlet channels). FIG. 4B shows that the distribution of coating alongthe axial direction is excellent, and it is clear that only alternatingchannels were coated (bright color) whereas the others remained uncoated(dark color).

FIGS. 5A-5F show SEM images of cellular ceramic articles after firing,which show the formation of a surface region on the coating layer havinga porosity distinct from the that of bulk ceramic. Specifically, FIGS.5A-C show a first substrate that has been coated on both sides ofchannel walls with a coating composition providing for relatively smallcoating layer pore size (“small pore size coating”) and FIGS. 5D-F showa second substrate that has been coated on both sides of channel wallswith a coating composition providing for somewhat larger coating layerpore size (“medium pore size coating”). Polished cross sectional viewsof the first substrate at two levels of magnification are shown in FIGS.5A-B and polished cross sectional views of the second substrate at thesame two levels of magnification are shown in FIGS. 5D-E. A top-downchannel view of the first substrate is shown in FIG. 5C and a top-downchannel view of the second substrate is shown in FIG. 5F. Differencesbetween surface and bulk porosity can be discerned by comparing polishedcross-section and top-down channel views. The interface between thecoating and substrate is difficult to distinguish providing reasonableevidence that the coating is well adhered.

The coating compositions for these examples are set forth in Table 1,wherein slurries were prepared by combining these as a powder withisopropanol. When preparing coating compositions, the ratio of powder tosolvent used is typically a function of the desired coating thicknessand the acceptable number of coating applications. Generally speaking,the more concentrated the slips are in solids, the thicker the resultingcoating and conversely, the more dilute the slips are in solids, thethinner the resulting coating. Preferred ranges for solidsconcentrations in the slips range from about 20% to about 75%, such asfrom about 35% to about 60%. Viscosity modification through addition ofa polymer may also be used to tailor thicknesses. In addition, use of aninorganic binder, such as that of an alcohol-dispersed colloidalboehmite, can result in a significant increase in the coating greenstrength. Additional exemplary binders include colloidal silica capableof being dispersed in IPA, tetraethylorthosilicate (TEOS),tetramethylorthosilicate (TMOS), and polyvinyl butyral. When using abinder that converts to inorganic material upon heating, the green slipchemistry should preferably be adjusted to account for this additionalinorganic to achieve the desired fired chemistry (AT, mullite, etc).

TABLE 1 Small pore size Medium pore size coating composition coatingcomposition Component Wt % Component Wt % Alumina 49.78 Alumina 49.78Titania 30.28 Titania 30.28 Silica 10.31 Silica 10.31 Strontiumcarbonate 8.09 Strontium carbonate 8.09 Calcium carbonate 1.35 Calciumcarbonate 1.35 Lanthanum oxide 0.19 Lanthanum oxide 0.19 Total 100.00Total 100.00 Rice starch (super 20.00 Corn starch (super 20.00 addition)addition)

FIGS. 6A and 6B show SEM images of top-down channel views of twosamples, wherein in the first sample, shown in FIG. 6A, a substrate wasnot coated with a coating layer and in the second sample, shown in FIG.6B, a substrate was coated on inlet channel surfaces with a coatinglayer as disclosed herein. Differences in surface porosity and pore sizedistribution were quantified through image analysis, where the surfaceof the uncoated substrate was found to have a total surface porosity of26.5% and the surface of a coated channel on the coated substrate wasfound to have a total surface porosity of 33.1%.

FIGS. 7A through 7C show SEM images of top-down channel views of firedceramics. FIG. 7A shows an uncoated ceramic channel. FIGS. 7B and 7Cshow the surfaces of coated ceramics. The ceramic coating in FIG. 7B wasgenerated from a slip having a 2:1 ratio of A10-325 alumina to isopropylalcohol (IPA). The ceramic coating in FIG. 7C was generated from a sliphaving a 2:1 ratio of solids to IPA, wherein the solids included about75% of A10-325 alumina and about 25% pore former (rice starch). Thechange in pore size and porosity is readily apparent.

The thickness of the coating layer can be altered by varying the slurryconcentration, slurry viscosity, and/or number of coats. For DPFapplications, the coating should preferably be as thin as possible whileminimizing deep bed filtration. For example, subsequent to a firingstep, the ratio of the thickness of the coating layer to a wall on thechannel on which it is coated can be least about 1:100, including atleast about 1:50, and further including at least about 1:20, such asabout 1:100 to 1:2, and further such as about 1:50 to 1:2, and yetfurther such as from about 1:20 to about 1:2, including from about 1:10to about 1:4, and further including from about 1:10 to about 1:5.

For example, for a wall thickness of about 200 microns, the thickness ofthe coating layer subsequent to a firing step can be at least about 2microns, including at least about 5 microns, and further including atleast about 10 microns, such as from about 2 to about 50 microns, andfurther such as from about 5 to about 50 microns, and yet further suchas from about 10 to about 50 microns.

In preferred embodiments, the minimum thickness of the coating layersubsequent to a firing step is about the same as the average pore sizeof pores in the coating layer. For example, if the pores in the coatinglayer have an average pore size of 5 microns, then the thickness of thecoating layer is preferably at least about 5 microns. If the pores inthe in the coating layer have an average pore size of 2 microns, thenthe thickness of the coating layer is preferably at least about 2microns.

In one set of preferred embodiments, the thickness of the coating layercan be approximately constant along the plurality of channels on whichit is coated. In another set of preferred embodiments, the thickness ofthe coating layer can vary along the axial length of least one of theplurality of channels. For example, the thickness of the coating layercan be at least 1.1 times greater, such as at least 1.2 times greater,further such as at least 1.5 times greater, and still further such as atleast 2 times greater at a first point along the axial length of thechannel than at a second point along the axial length of the channel.

In yet another set of preferred embodiments, the thickness of thecoating layer can vary along different channels. For example thethickness of the coating layer can be greater along channels that arenearer to the central longitudinal axis of the cellular ceramic bodythan those that are farther (i.e., more radially outward) from thecentral longitudinal axis of the cellular ceramic body, such that thethickness of the coating layer most thickly coated on a channel is atleast 1.1 times greater, and further such as at least 1.2 times greater,and still further such as at least 1.5 times greater, and still yetfurther such as at least 2 times greater than the thickness of thecoating layer most thinly coated on a channel.

The thickness of the coating layer can also be expressed in terms of aclosed frontal area increase. Preferably, the coating should provide fora closed frontal area increase of at least 1%, such as from 1-25%, andfurther such as from 1-10%, including from 2-10%. For example, for apart having 300 cells per square inch with 8 mil thick walls, a 1%increase in closed frontal area corresponds to about a 2 micron thickcoating layer.

The thickness of the coating layer may also vary along the axial lengthof least one of the plurality of channels such that the closed frontalarea is at least 5%, such as at least 10%, and further such as at least20% greater at a first point along the axial length of the channel thanat a second point along the axial length of the channel.

Exemplary embodiments disclosed herein include those in which thecoating layer extends along the entire length of the channel or channelson which it is coated or extends only partially along the length of thechannel or channels on which it is coated. Exemplary embodiments alsoinclude those in which the coating layer extends along the entire lengthof at least one channel and only partially extends along the length ofat least one other channel.

Subsequent to a firing step, the surface porosity of the coating layercan be the same or different than the surface porosity of a wall on thechannel on which it is coated. Differences in surface porosity can bequantified, for example, through image analysis. For example, thesurface porosity of the coating layer can be at least 5% greater, suchas at least 10% greater, and further such as at least 20% greater, andyet further such as at least 30% greater, and still yet further such asat least 50% greater than the surface porosity of a wall on the channelon which it is coated. The surface porosity may also be at least 5% lessthan, such as at least 10% less than, and further such as at least 20%less than, and yet further such as at least 30% less than, and still yetfurther such as at least 50% less than the surface porosity of a wall onthe channel on which it is coated.

Subsequent to a firing step, the average pore diameter of pores in thecoating layer can be the same or different as the average pore diameterof pores in a wall on a channel on which it is coated. Average porediameter can be measured, for example, by mercury porosimetry. Forexample, the average pore diameter of pores in the in the coating layercan be at least 5% smaller, such as at least 10% smaller, and furthersuch as at least 20% smaller, and yet further such as at least 30%smaller, and still yet further such as at least 50% smaller than theaverage pore diameter of pores in a wall on the channel on which it iscoated. The average pore diameter of pores in the coating layer may alsobe at least 5% larger, such as at least 10% larger, and further such asat least 20% larger, and yet further such as at least 30% larger, andstill yet further such as at least 50% larger than the average porediameter of pores in a wall on the channel on which it is coated.

FIG. 8 illustrates noise to signal ratio upon soot loading (denotedN/S). The signal (S) in this case is the pressure drop at 5 g/L sootloading, while the noise (N) is the pressure drop associated with deepbed filtration, such that a larger noise-to-signal ratio indicates ahigher amount of deep bed filtration.

FIG. 9A shows pressure drop data for 2″×6″ plugged DPF-style filters at26.5 scfm and room temperature as function of the soot load (Printex U)while FIG. 9B shows the decrease in N/S upon coating for these parts.

FIGS. 10A and 10B compare pressure drop (FIG. 10A) and deep bedfiltration (FIG. 10B) for coated and uncoated 2″×6″ filters. As can beseen in FIGS. 10A and 10B, the uncoated filter had significant deep bedfiltration which was reduced in the coated filter. As can be seen fromFIGS. 10A and 10B, the coating in these examples appears to raise theclean pressure drops while lowering the soot-loaded pressure drops.

Accordingly, subsequent to firing and plugging steps, the cellularceramic article preferably exhibits a decrease in the soot-loadedpressure drop noise to signal ratio compared to an uncoated filter. Incertain preferred embodiments, subsequent to firing and plugging steps,the cellular ceramic article exhibits a pressure drop upon soot loadingnoise to signal ratio of less than about 0.4, such as less than about0.35, and further such as less than about 0.3, such as from 0.2 to about0.4, and further such as from about 0.2 to about 0.35, and still furthersuch as from about 0.2 to about 0.3.

FIG. 11 plots the mass-based filtration efficiency for membrane-coated Aand uncoated B filters. The efficiency is not only higher at everysoot-loading with a membrane, but it also reaches 100% efficiency at alower soot-loading than the uncoated filters.

FIG. 12 shows results from test performed in laboratory regenerationexperiments with 2″×6″ filters coated over different lengths. Includedare reference parts without coating as well as two sets in which thefilter has been coated over ⅓ and ⅔ of the length with a coating. In theexperiments, the soot load (Printex U) is successively increased andthen a “worst case” regeneration is performed, resulting in extremetemperatures and temperature gradients inside the filter. Thetemperatures and gradients are measured through several thermocouplesinstalled inside the filter (Type K and Type S thermocouples are useddepending on the temperature range). As the soot load increases, thetemperatures and gradients increase. The integrity of the filter isevaluated after each test and testing is stopped when damage, i.e.cracking, is observed. The results shown indicate the last survived andthe first failed temperatures and gradients. Two samples are used foreach filter condition. It can be seen that both coated candidatesperform similarly to the reference samples, with the parts coated overonly ⅓ being slightly lower in failure conditions. Overall, thissuggests that the coating does not result in compromisedthermo-mechanical properties.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method of manufacturing a cellular ceramicarticle comprising: providing a green cellular ceramic body comprising abinder material and a plurality of channels; and coating at least one ofthe plurality of channels that is intended to be plugged as an inletchannel with a slurry comprising a green coating composition and asolvent to form a coating layer on at least one of the plurality ofchannels that is intended to be plugged as an inlet channel; wherein thebinder material is insoluble in the solvent and the at least one of theplurality of channels is unplugged when coated with the slurry; andwherein, subsequent to a firing step, the average pore diameter of poresin the coating layer is at least 5% smaller than the average porediameter of pores in a wall on the channel on which it is coated.
 2. Themethod of claim 1, wherein the solvent comprises at least one of analcohol, ester alcohol, ester, hydrocarbon, aldehyde, ketone, andcarboxylic acid.
 3. The method of claim 1, wherein the green cellularceramic body comprises at least one material that upon firing reacts toform at least one material selected from the group consisting ofaluminum titanate (AT), metals, intermetallics, mullite, cordierite,alumina (Al₂O₃), silicon carbide (SiC), silicon nitride (Si₃N₄), siliconaluminum oxynitride (SiAlON), and zeolites.
 4. The method of claim 1,wherein the binder material comprises at least one cellulose ether. 5.The method of claim 1, wherein the green coating composition comprisesat least one material that upon firing reacts to form at least onematerial selected from the group consisting of aluminum titanate (AT),metals, intermetallics, mullite, cordierite, alumina (Al₂O₃), zircon,alkali and alkaline-earth alumino-silicates, spinels, persovskites,zirconia, ceria, silicon carbide (SiC), silicon nitride (Si₃N₄), siliconaluminum oxynitride (SiAlON), and zeolites.
 6. The method of claim 1,wherein the step of coating at least one of the plurality of thechannels with the slurry comprises at least one method selected frompouring the slurry down at least one of the channels (waterfall method),dipping the green cellular ceramic body in the slurry, using a vacuum topull the slurry on the channels, and pumping the slurry onto thechannels.
 7. The method of claim 1, wherein, subsequent to a firingstep, the ratio of the thickness of the coating layer to a wall on thechannel on which it is coated is at least about 1:100.
 8. The method ofclaim 1, wherein, subsequent to a firing step, the thickness of thecoating layer varies along the axial length of least one of theplurality of channels such that the closed frontal area is at least 5%greater at a first point along the axial length of the channel than at asecond point along the axial length of the channel.
 9. The method ofclaim 1, wherein, subsequent to firing and plugging steps, the cellularceramic article exhibits a decrease in the soot-loaded pressure dropnoise to signal ratio compared to an uncoated filter.
 10. The method ofclaim 1, wherein the cellular ceramic article exhibits an increase infiltration efficiency at a given soot-loading compared to an uncoatedfilter.